A.R. Tagilaev, M.Д. Isaev, А.I. Egizbaev, R.Y. Aliev, P.V. Talaev

Radio transmitters expose to many different unsettling influences more critical of those are unmatched load, overload of RF input, thermal overload, instability of supply voltage and others. Therefore continuous and stable working of radio transmitter depends on safety of power amplifier. The main purpose of article is consideration of digital method power amplifier characteristics stabilization. This method is based on using digital microcontroller and different sensors for real time diagnostic of errors. This information use for generation signals for power amplifier control. In compliance with structure of power amplifier input RF signal come at attenuator with digital control witch allow to change input level and respectively output level too. Bias voltages for power transistors sets with digital potentiometers. To control bias current use current sensors mounted in supply circuits. The reference values of bias current from sensors determined at tuning stage and stored in nonvolatile memory of microcontroller. Digital potentiometers allow set operating mode of transistors using stored reference values. Radio transmitter also contain temperature sensor for correct output power and bias voltages when it is in continuous power mode. Temperature data used for current sensor calibration. Directional coupler follows the power amplifier. It is forming two signals for incident wave (Uinc) and for reflected wave (Uref). This signals use to control output power and VSWR. Values of Uinc and Uref for specified power stored in nonvolatile memory of microcontroller as reference values. When output power of transmitter deviate from normal value microcontroller form control command for input attenuator to correct it based on Uinc and Uref data. Uinc and Uref also allow to calculate VSWR, check antenna matching and form error signal when antenna is unmatched. Using of microcontroller make easy connection between power amplifier and PC via Ethernet and others digital interfaces. Based on discussed method we design UHF band power amplifier with following time characteristics: output power correction time (amplitude-frequency characteristic ripple 5 dB) (max) 60 us; time to shutdown power with short-circuit antenna (max) 50 us; time to output power setup after turn on (max) 50 ms. Communication between amplifier and PC implement via RS232 or RS485 interfaces. This amplifier structure may expand with signal generator and RF filter-switch to design complete wide band RF transmitter. Applying of considered method of digital stabilization allow to enhance safety and quality of RF systems.